The plane, a Horizon Air Q400, took off from Seattle-Tacoma International Airport and came down after what was called an “unauthorised take-off” by authorities.
It is not known when the man, believed to be a 29-year-old local airline employee, survived the crash.
Two F15 fighter jets has been pursuing the plane when it came down.
Pierce County Sheriff Paul Pastor told reporters the incident was not terror related.
It appeared to be a joyride gone wrong, he said, adding “most terrorists do not do loops over the water”.
Air traffic controllers tried to encourage the man to land before the plane crashed.
Some dude stole a plane from #Seatac (Allegedly), did a loop-the-loop, ALMOST crashed into #ChambersBay, then crossed in front of our party, chased by fighter jets and subsequently crashed. Weird times. pic.twitter.com/Ra4LcIhwfU
— bmbdgty (@drbmbdgty) August 11, 2018
The Q400 is a twin-propeller aeroplane with up to 78 seats, depending on configuration.
In a statement, Seattle-Tacoma International Airport said: “An airline employee conducted an unauthorized take-off without passengers at Sea-Tac; aircraft has crashed in south Puget Sound.”
Normal operations at Sea-Tac Airport have now resumed.